The present invention relates to a novel insulating joint with electrically separated part for rubber or plastic insulated power cables.
In the sheath of a single conductor cable, electromagnetic induction of a conductor current involves a potential which causes a sheath circuit loss. However, such a potential may be lowered by the so-called crossbond method. The crossbond method uses an insulating joint between adjacent cables. In the insulating joint, the insulation shield is electrically insulated by some means.
An example of the electrically separated part of the insulating joint for rubber or plastic insulated power cables is shown in FIG. 1. Referring to FIG. 1, insulation shields 5 are coaxially formed around two jointed conductors 1 and 1, thus defining an electrically separated part 8 which separates the insulation shields 5 in the vertical direction. Such an insulating joint of such a crosslinked polyethylene insulated cable is manufactured in the following manner.
In order to join the ends of two cables, cable insulations 3 are tapered as shown in FIG. 1. After the conductors 1 and 1 are spliced with a compression sleeve or the like, a conductor shield 2 of a semiconductive type or a semiconductive thermally-shrinkable tube is formed thereover. After winding a rubber or plastic insulating tape such as a crosslinkable insulating tape around the conductor shield 2, the resultant structure is heated under pressure for bonding. Alternatively, a joint insulation layer 4 is formed on the conductor shield 2 by placing a suitable mold (not shown) around the conductor shield 2, injecting or extruding a melted resin into the mold, and adhering them together by heating by a suitable means. Insulation shields 5 defining an electrically separated part 8 therebetween are coaxially formed around the joint insulation layer 4, thus completing manufacture of a joint.
However, a conventional insulating joint of this type for a rubber or plastic insulated cables has many drawbacks as listed below:
(1) When the layer of the wound tape is heated for adhesion, the distal ends of the insulation shields 5 defining the electrically separated part 8 therebetween are deformed. Since the electric field acting on these distal ends therefore increases, the resultant joint will easily break down at these distal ends.
(2) When the electrically separated part 8 is defined by the insulation shields 5, it is hard to form the insulation shields 5 coaxially and concentrically. Therefore, disturbance in the electric field easily occurs.
In order to eliminate these drawbacks, it is proposed, as shown in FIG. 2, to form a layer 7 with high dielectric constant and high volume resistivity on a joint insulation layer 4 which covers the joint portion of conductors of rubber or plastic insulated cables. The layer 7 has a volume resistivity, at an AC voltage of commercial frequency, of 10.sup.6 to 10.sup.12 .OMEGA..multidot.cm and a specific dielectric constant of 6 to 100. The layer 7 thus defines an insulating joint of high dielectric constant and high volume resistivity type which electrically separates insulation shields 6. If the volume resistivity of layer 7 is below 10.sup.6 .OMEGA..multidot.cm and the specific dielectric constant of the layer 7 exceeds 100, respectively, flashover occurs due to an impulse voltage which may enter the cable. On the other hand, if the volume resistivity of layer 7 exceeds 10.sup.12 .OMEGA..multidot.cm and the specific dielectric constant of the layer 7 is below 6, respectively, electrical stress concentration occurs upon application of a load on the cable, easily leading to breakdown.
In a conventional electrically separated part of high dielectric constant and high volume resistivity type, method is proposed in which a layer of high volume resistivity within the range as defined above is formed around an insulator by suitably adjusting the carbon content. According to this method, the layer with high volume resistivity containing only carbon causes variations in volume resistivity upon being subjected to a thermal history such as a heat cycle based on a cyclic loading.